(1) Field of the Invention
The present invention relates to an article having a substrate of lithium niobate single crystal, especially to an optical waveguide substrate, and a process for producing the same.
(2) Related Art Statement
Lithium niobate (LiNbO.sub.3) single crystal and lithium tantalate (LiTaO.sub.3) single crystal have been expected as materials for optoelectronics. A liquid phase epitaxial process is expected as a process for producing thin films of the single crystal with excellent crystallinity and quality, because it is possible to produce the thin films at a temperature lower than a temperature at which the single crystals are produced by a so called "Czochralski" process.
For example, according to the description in "Journal of Crystal Growth", 46, (1979), pp 314-322, a film of lithium niobate-lithium tantalate solid solution is formed on a substrate of lithium tantalate single crystal by the liquid phase epitaxial process. According to the description in "Appl. Phys. Letters", Vol. 26, No.1, (1975), pp 8-10, a thin film of lithium niobate single crystal is formed on a substrate of lithium tantalate single crystal by the liquid phase epitaxial process. According to the description in "Journal of Crystal Growth", 50, (1980), pp 291-298, a thin film of lithium niobate single crystal is formed on a substrate of lithium tantalate single crystal by so called "Capillary Liquid Epitaxial" process. However, this process based on a principle different from that of an ordinary liquid phase epitaxial process. Moreover, the document does not disclose the formation of a film of lithium niobate- lithium tantalate solid solution.
The film-forming method in the liquid phase epitaxial process will be schematically explained. First, lithium niobate (a solute) and LiVO.sub.3 (a melting medium) are charged and mixed together. A saturation temperature corresponding to a melt having the charged composition is taken as "T0". While a temperature of the melt is held at T1 higher than the saturation temperature T0, lithium niobate and LiVO.sub.3 are uniformly melted. Then, the melt is led to supercooled state by lowering the temperature of the melt to a temperature lower than the saturation temperature T0. A substrate is contacted with the supercooled melt.
The inventors considered the following process for producing optical waveguide devices with high quality. In the process, a single crystal film is formed on a single crystal substrate by the liquid phase epitaxial process to provide the film having a refractive index higher than that of the substrate and the single crystal film is utilized as an optical waveguide.
A refractive index of lithium tantalate is lower than that of lithium niobate. Lithium niobate and lithium tantalate may be mixed with each other in any proportion to produce solid solution. The larger the content of tantalum in the solid solution, the lower the refractive index thereof. Therefore, a substrate of lithium tantalate single crystal has been used as a substrate on which a film of the solid solution or lithium niobate is produced by the liquid phase epitaxial process. Because the refractive index of lithium tantalate is lower than that of the solid solution or lithium niobate and when the film of the solid solution or lithium niobate is produced on the substrate of lithium tantalate single crystal, the film may be utilized as the optical waveguide.